Seriously, that rosette is a different species... if it has died out while the rest of the plant persists, that very strongly supports it too... Likely a seed that was in the pot from the grower, assuming that you are not growing any Draba sp..
Take a gander at the Draba sp. sold by Wild Ginger Farm; Draba lasiocarpa looks like a possibility, at least among those in their catalogue (they are probably growing others that are not in the catalogue).
Yes, many Androsace sp. are very hardy and make great rock garden plants for zone 3.
